It is well known that incandescent light bulbs are very energy inefficient light sources—about 90% of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are by a factor of about 10 more efficient, but are still less efficient than a solid state semiconductor emitter, such as light emitting diodes, by a factor of about 2.
In addition, incandescent light bulbs have a relatively short lifetime, i.e., typically about 750 to 1000 hours. Fluorescent bulbs have a longer lifetime (e.g., 10,000 to 20,000 hours) than incandescent lights, but they contain mercury, not an environment friendly light source, and they provide less favorable color reproduction. In comparison, light emitting diodes have a much longer lifetime (e.g., 50,000 to 75,000 hours). Furthermore, solid state light emitters are very environmentally “green” light sources and they can achieve very good color reproduction.
Accordingly, for these and other reasons, efforts have been ongoing to develop solid state lighting devices to replace incandescent light bulbs, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide improvement with respect to energy efficiency, color rendering index (CRI Ra), luminous efficacy (lm/W), color temperature, and/or duration of service, especially for indoor applications.
A semiconductor light emitting device utilizing a blue light emitting diode having a main emission peak in blue wavelength range from 400 nm to 490 nm, and a luminescent layer containing an inorganic phosphor that absorbs blue light emitted by the blue LED and produces an exciting light having an emission peak in a visible wavelength range from green to yellow (in the range of about 525 nm to 580 nm) with spectrum bandwidth (full width of half maximum, simply refer to FWHM) about 80 to 100 nm.
Almost all the known light emitting semiconductor devices utilizing blue LEDs and phosphors in combination to obtain color-mixed light of the emission light from the blue LEDs and excitation light from the phosphors use YAG-based or silicate-based luminescent layer as phosphors. Those solid state lighting devices have typically white color temperature about 5000 K to 8500 K with low color rending index Ra about 60˜70. This white solid state lighting device is not desirable for some applications, like indoor applications, which require warm white color about 2700 K to 3500 K with a high color rending index Ra above 80.
Known warm white semiconductor light emitting solutions and their low luminous efficacy issues are shown at the followings:    1. Blue LED with mixture YAG-based or silicate-based phosphors (for exciting yellow light) and nitrides or sulfides phosphors (for exciting red light) for a warm white light. YAG-based or silicate-based phosphors excite a broad-band yellow light having a full spectrum range from 500 nm to 650 nm with FWHM about 80˜100 nm. But this yellow excitation light has a shortage in red and bluish green wavelength range, which limits its color rendering index Ra less than 70. Adding a red phosphor to the yellow phosphor can compensate for a shortage of red light, resulting in improved color rendering index about 75˜80. But the red phosphor absorbs the emission blue light (with a peak wavelength around 460 nm) and excites a red light (with a peak wavelength around 620 nm), which causes a significant Stoke-shift issue in photonic energy loss. Another issue with the mixture of yellow and red phosphors is the broad-band spectrum distribution of the excitation light, where luminous flux contribution is low at two edge spectrums range due to the low sensitivity of red and bluish green wavelength light to the human eye.    2. Blue LED with mixture YAG-based or silicate phosphors (for exciting green light) and nitrides or sulfides phosphors (for exciting orange light) for a high color rendering warm white light. The mixture of green and orange phosphors can compensate for the shortage of red light and bluish green light, resulting in warm white with high color rendering index above 80. But it has three issues which will cause low luminous efficacy: a) multi-phosphors self-absorption loss of the photons excited from the green and orange phosphor particles; b) Stoked-shift loss from blue-to-red wavelength conversion; c) low luminous flux contribution from the red and bluish green wavelength in the broad-band spectrum distribution edge of the excitation light.    3. Blue LED with YAG-based or silicate-based phosphors (for exciting yellow light or blue shifting yellow light) and mixing with a semiconductor emitting red/amber color light for a high color rendering warm light. Adding red/amber semiconductor emitters directly to the solid state white lighting device can solve the issues of multi-phosphors self-absorption loss and Stokes shift loss of the blue-to-red wavelength conversion. But it still suffers from a low luminous flux contribution issue from the red and bluish green wavelength range in the broad-band spectrum distribution of the excitation light. And it still has a shortage of bluish green wavelength. Besides this, more efforts are ongoing to improve the light mixture from the multi-color semiconductor light emitters.